The scale on a map is 9cm : 4km. If the distance between two cities is 8km, how far apart in cm are the two cities on the map?
The scale means that every 9cm on the map corresponds to an actual distance of 4km. An actual distance of 8km is the same as 2 $\cdot$ 4km. The distance between the two cities on the map is 2 $\cdot$ 9cm, or 18cm.